Why? The vast majority of the requests they serve are cached at multiple layers above the DB. Switching to an entirely different DB would require rewriting a lot of code, a much larger learning curve to contribute (MediaWiki is FOSS), and limit its use to large sites (MediaWiki is widely used by small sites).
I'm not familiar enough with their data structure to comment, but the concern would seem to be around write performance... if they have to use sharding in an rdbms like my/mariadb, then it's not too much harder to change databases at that point.. not to mention improvements with distributed reads on a cache miss..
